1. Field of the Invention
The present invention relates to an optical member for vacuum ultraviolet used as an optical part (or component) in a light-source optical system, a projection optical system, etc. of an aligner for fabricating semiconductor devices such as microprocessors, memories, system LSIs, image sensors, light-emitting devices, display devices, and so on, and an aligner and a device manufacture method using the same.
2. Related Background Art
The excimer lasers are drawing attention as the only high-power laser that oscillate in the ultraviolet region and application thereof is expected in electronic industries, chemical industries, and energy industries. Specifically, they are utilized in the processing of metal, resin, glass, ceramics, semiconductors, and so on, in chemical reactions, etc. Recently, considerable progress has been made in their application as light sources of an aligner for ultra-fine lithography, taking advantage of their short wavelength property.
Currently dominating lithography steps are methods of transferring a pattern drawn on a mask, onto a wafer through lenses (photolithography). In general, the resolution of the transfer pattern increases in proportion to each of the numerical aperture (NA) of lenses and the inverse of the wavelength of light. However, the numerical aperture of lenses involves manufacturing problems, and it is thus effective to shorten the wavelength of the light in order to enhance the resolution. For this reason, the light sources for photolithography have been decreasing their wavelengths, e.g., from the g-line (436 nm) to the i-line (365 nm), and further to the KrF excimer laser (248 nm).
Particularly, the resolution of 0.23 xcexcm has been achieved in a reduction projection aligner using the KrF excimer laser as a light source.
It has been considered heretofore that synthetic quartz glass (silica glass), which demonstrates little absorption and which can be polished in a large diameter, is suitable as an optical material for the lenses and others in an apparatus using as a light source the KrF excimer laser, or light of a shorter wavelength than it, particularly, light of a wavelength not more than 200 nm, i.e., light of the so-called vacuum ultraviolet region. As a crystal for such use, there have been proposed a lithium fluoride crystal, a magnesium fluoride crystal, and a calcium fluoride crystal (fluorite). For a general review, see for instance, Japanese Patent Application Laid-Open Nos. 10-279378, 11-021197, 9-315894, 10-330114, 11-228292, and 2000-191322.
However, according to the above prior art technologies, for example, the lithium fluoride crystal has an extreme deliquescence nature so as to be difficult to polish and is therefore not suitable for practical use. Further, the magnesium fluoride crystal is a biaxial crystal and has the property of optical anisotropy, thus causing the double refraction (birefringence) phenomenon. For this reason, the magnesium fluoride crystal can be used for polarizing elements such as polarizing prisms or the like utilizing the birefringence phenomenon or for optical members that do not have to have high imaging performance, such as window members for vacuum system, but is not an appropriate material for optical parts requiring high imaging performance, such as lenses, prisms, and the like used in the photolithography.
The calcium fluoride crystal (fluorite) is an excellent UV-transmissive material free from deliquescence nature and optical anisotropy, and has therefore been considered as a promising material that can be used for precision optical systems. However, the fluorite has a problem of a high cleavage nature.
The absorption of light becomes difficult with increase of the bandgap. The following equation is an energy reduction equation of a photon.
xe2x80x83E(eV)=1240/xcex (nm)
As seen from the equation, it is necessary to use a material with a wide bandgap of not less than 10 eV in order to prevent absorption at the wavelengths of not more than 120 nm in the vacuum ultraviolet region. Then, optimal crystals are those having the crystal structure of the cubic crystal system free from optical anisotropy. Although the melting point of the conventional fluorite is about 1400xc2x0 C., preferred crystals for practical use are those having a melting point of not more than 1000xc2x0 C.
A primary object of the present invention is to provide a high-quality optical member for vacuum ultraviolet, free from deliquescence nature and cleavage nature, with ease of processing and high practicality, and with little absorption, and also to provide an aligner and a device manufacture method using the optical member.
According to one aspect of the present invention, there is provided an optical member for vacuum ultraviolet comprising a substrate obtained by processing a crystal of AMF3-type perovskite structure.
According to another aspect of the present invention, there is provided an optical member comprising a substrate and an AMF3 coating obtained using a crystal of AMF3-type perovskite structure as an evaporation source.
In one preferred embodiment, A is any one of Li, Na, and Cs and M is any one of Ca, Sr, Ba, Mg, and Zn.
In another preferred embodiment, A is K and M is Ba or Zn.
The inventor has discovered that when a single crystal of the AMF3-type perovskite structure is grown from a 1:1 mixture of an alkali halide of LiF or the like and a compound of MgF2, CaF2, or the like, the crystal is an excellent optical material of the cubic crystal system free from optical anisotropy, with low birefringence, with little absorption in the vacuum ultraviolet region, and free from deliquescence nature and cleavage nature and can be used for the optical member of an aligner or the like.
Namely, by using an optical member such as a lens, a mirror, a prism, a window member, etc. comprising the substrate obtained by processing the above crystal, in a projection optical system or light-source optical system of an aligner using a light of the vacuum ultraviolet region as an illumination light, photolithography advanced in microprocessability can be implemented.
Further, in the present invention, evaporated particles generated using the crystal of the AMF3-type perovskite structure as an evaporation source may be deposited on a surface of a lens, a mirror, a prism, a window member, or the like to form a coating comprised of an AMP3 fluoride crystal.